1. Field of the Invention
This invention relates to soft-start circuits used in switching power regulators, and more particularly to soft-start circuits using combinations of reference voltages and programmable width burst pulses for controlling output voltages of switching power regulators during start-up.
2. Description of the Related Art
One type of switching power regulator often used to perform DC-to-DC voltage conversion is a step-down regulator, which generally operates to convert a higher voltage (e.g. 12V) to a lower value as required by one or more load devices. Switching power regulators often use two or more power transistors to perform the input voltage to output voltage conversion. One common example of such a switching power regulator, commonly called a “Buck regulator”, implemented with MOS devices is shown in FIG. 1a. Regulator 100 may be configured to operate in either PWM (pulse width modulation) mode or PFM (pulse-frequency modulation) mode, switching a P-channel device 107 and an N-channel device 109 in order to produce a square-wave at their common node LX. The produced square-wave can be smoothed out using an LC circuit comprising inductor 111 and load capacitor 113 to produce a desired output voltage, Vout. A control loop, comprised of an error amplifier 115 and a control logic block 101 can be configured to control the output square-wave, thereby controlling switching P-channel device 107 and N-channel device 109, and hence the resulting value of Vout. In general, P-channel device 107 and N-channel device 109 are controlled such that they do not conduct current at the same time. Typically, when P-channel device 107 is turned on (Vg_P is logic 0), N-channel device 109 is turned off (Vg_N is logic 0), and when P-channel device 107 is turned off (Vg_P is logic 1), N-channel device 109 is turned on (Vg_N is logic 1). IL represents the load current flowing in inductor 111. When operating in PFM mode, P-channel device 107 is turned on at a frequency and duty cycle that is a function of an input voltage Vin, the output voltage Vout, and the value of inductor 111. While in PFM mode, N-channel device 109 is turned off to optimize efficiency by reducing gate charge dissipation. Vout is regulated by skipping switching cycles that turn on P-channel device 107.
In order to maintain proper operation of a load device coupled to regulator 100, and often to limit an input current developed in regulator 100 while charging load capacitor 113, Vout is typically ramped from its initial value (called its pre-bias value) to a desired output voltage at a controlled rate. This rate may be chosen as required by the load device and the designed maximum input current allowed for charging load capacitor 113. Often, the pre-bias value is at or near 0 volts. If that is the case, regulator 100 generally attempts to ramp its output Vout from 0V to the desired voltage in a predetermined amount of time. Therefore, some switching power regulators also include soft-start circuits to limit input current during start-up. For example, Texas Instruments' TPS6205x synchronous step-down regulator features an internal soft-start circuit that limits the inrush current during start-up to prevent possible voltage drops of the input voltage if a battery or a high impedance power source is connected to the input. The soft-start circuit within the TPS6205x is implemented as a digital circuit increasing the switch current in steps of 200 mA, 400 mA, 800 mA, and then the typical switch current limit of 1.2 A. A typical start-up time with a 22 μF load capacitor and a 200-mA load current would be 1 ms. As a result, however, the start-up time mainly depends on the load capacitor and load current, with the regulator requiring an extra pin for the external components.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.